1. Field of the Invention
This invention relates to a system and apparatus for improving blood flow in the heart, and more particularly to a system and apparatus for performing transmyocardial revascularization or stimulating angiogenesis using a resistive heater.
2. Description of Related Art
Heart disease is a common medical problem in developed countries. The major cause of heart disease in developed countries is impaired blood flow to the heart. The coronary arteries which supply blood to the heart become narrowed due to a disease known as atherosclerosis and as a result, part of the heart muscle is deprived of oxygen and other nutrients. The resulting condition known as ischemia can lead to angina pectoris, a pain in the chest, arms or jaw due to a lack of oxygen to the heart, or the infarction or death of an area of the myocardium caused by the ischemia.
Techniques to supplement the flow of oxygenated blood directly from the left ventricle into the myocardial tissue have included needle acupuncture to create transmural channels, implantation of T-shaped tubes into the myocardium and the like. Efforts to graft the omentum, parietal pericardium, or mediastinal fat to the surface of the heart have had limited success. Others have attempted to restore arterial flow by implanting the left internal mammary artery into the myocardium.
Coronary artery blockage can be treated with a variety of different modalities. Drug therapy is used to dilate the arteries and dissolve clots. Examples of medicaments used in dilation include nitrates, beta-blockers and peripheral vasodilatator drugs. Transluminal angioplasty is performed by inflating a balloon at a narrowed or clogged site in the artery. When drug therapy is ineffective or angioplasty is too risky, coronary artery bypass grafting (CABG) may be performed. CABG is a major surgical procedure requiring opening the chest and the use of a heart-lung machine.
Another method of improving myocardial blood supply is transmyocardial revascularization (TMR) where channels are formed from the epicardial to the endocardial portions of the heart. MR relieves ischemia by allowing blood to pass from the ventricle through the channels either directly into other vessels perforated by the channels or into myocardial sinusoids which connect to the myocardial microcirculation. By analogy, TMR has been compared to transforming the human heart into one functionally resembling that of a reptile with respect to myocardial blood flow.
In the reptilian heart, blood flow occurs via communicating channels between the left ventricle and the coronary arteries (Frazier, O. H.: Myocardial Revascularization with Laser--Preliminary Findings, Circulation, 1995; 92 [suppl II]:II-58-II-65). There is evidence of these communicating channels in the developing human embryo. In the human heart, myocardial microanatomy includes the presence of myocardial sinusoids. These sinusoidal communications vary in size and structure, but represent a network of direct arterial-luminal, arterial-arterial, arterial-venous, and venous-luminal connections. This vascular mesh forms an important source of myocardial blood supply in reptiles but its role in humans is poorly understood.
Numerous studies have been performed on TMR methods using lasers to create channels in the myocardium. These studies have demonstrated histological evidence of probable new vessel formation (a process known as angiogenesis) adjacent to collagen occluded transmyocardial channels. In contrast, studies of myocardial acupuncture or boring, (mechanically displaces or removes tissue), showed acute thrombosis followed by organization and fibrosis of clots as the principal mechanism of channel closure.
U.S. Pat. No. 4,658,817 discloses a method and apparatus for TMR using a laser. A surgical CO.sub.2 laser includes a handpiece for directing a laser beam to a desired location. Mounted on a forward end of the handpiece is a hollow needle to be used in surgical applications where the needle perforates a portion of tissue to provide the laser beam direct access to distal tissue.
U.S. Pat. No. 5,125,926 (the "'926 Patent") teaches a heart-synchronized pulsed laser system for TMR. In the '926 Patent, contraction and expansion of a beating heart are monitored. During monitoring, the apparatus triggers the delivery of a pulse of laser energy to the heart during a predetermined portion of the heartbeat cycle. This heart-synchronized pulsed laser system is important where the energy and pulse rate of the particular type of laser are potentially damaging to the beating heart or it's action. Application of laser energy to a beating heart can induce fibrillation or arrhythmia. Additionally, as the heart beats, the spatial relationship between the heart and the tip of the laser delivery probe may change so that the necessary power of the beam and the required position of the handpiece may be unpredictable.
U.S. Pat. Nos. 5,380,316 (the "'316 Patent") and 5,389,096 (the "'096 Patent) both disclose respectively, systems and methods for intra-operative and percutaneous myocardial revascularization. The '316 Patent is related to TMR performed by inserting a portion of an elongated flexible lasing apparatus into the chest cavity of a patient and lasing channels directly through the outer surface of the epicardium into the myocardium tissue. In the '096 Patent, TMR is performed by guiding an elongated flexible lasing apparatus into a patient's vasculature such that the firing end of the apparatus is adjacent to the endocardium. Channels are created directly through the endocardium into the myocardium tissue without perforating the pericardium layer.
The use of lasers as the energy source in TMR has deficiencies. Lasers can be very expensive energy sources. Also, those lasers which permit acceptable depths of tissue necrosis provide a low volumetric ablation rate.
RF energy has also been disclosed as an alternative energy source for TMR as described in U.S. Pat. No. 5,683,366 (the "'366 Patent"). In the '366 Patent, a probe is introduced into a thoracic cavity of a patient through a percutaneous penetration, a thoracotomy or a sternotomy. An RF electrode is positioned adjacent to a wall of the patient's heart. An electrically conducting liquid is directed to the heart wall to provide a current flow path. High frequency voltage is applied to ablate or otherwise disintegrate tissue at the heart wall. The probe is then axially translated towards the ventricular wall to form a revascularizing channel or artificial vessel from the ventricle to the myocardium in order to increase blood flow.
One drawback of many RF devices used for tissue ablation is an inability to control the depth of necrosis (e.g. cell death) in the tissue being treated. Most electrosurgical devices rely on the creation of an electric arc between the treating electrode and the tissue being cut or ablated to cause the desired localized heating. Such arcs, however, often create very high temperatures causing a depth of tissue necrosis greater than 500 .mu.m, frequently greater than 800 .mu.m, and sometimes as great as 1700 .mu.m. The inability to control such depth of tissue necrosis is a significant disadvantage in the use of RF energy for TMR applications.
PTC Application WO 96/35469 discloses the use of lasers, a rotating auger device, a circular cutting device, a high velocity fluid jet and resistive heating device as channel forming devices. The limitations of lasers are discussed herein. The cutting and fluid devices present the risk of coronary and cerebral embolisms from small pieces of dislodged tissue causing emboli that lodge in a coronary or cerebral artery. While disclosed embodiments of the resistive heating device have the limitation of not being able to precisely control the depth of penetration into coronary tissue.
There is a need for a TMR method and apparatus which uses a relatively simple energy source. There is a further need for a TMR energy source which provides both localized and controllable heating. Yet there is a further need for a method and apparatus which use resistive heating to create revascularization channels and/or stimulate angiogenesis. Still a further need exists for a method and apparatus using resistive heating to create revascularization channels and/or stimulate angiogenesis by piercing a heart wall prior to the delivery of thermal energy from the resistive heating source. Still yet another need exists for a method and apparatus using resistive heating to create revascularization channels and/or stimulate angiogenesis by heating the resistive heating source prior to piercing a heart wall.